I claim the ornamental design for a guitar case, as shown and described.
FIG. 1 is a front perspective view of the guitar case, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines in the drawings of the guitar case are for showing the rear and side portions of the guitar case and form no part of the claimed design.